Practical problems in spectroradiometry are beginning to overtax the traditional techniques for establishing standard spectral power distributions based on the use of one or other of the four optical radiation scales: theOptical Pyrometer Scale, theColor Temperature Scale, thePhotometric Intensity Scaleand theRadiometric Scale. Proposals are therefore being made for changes in this existing system of scales which, because of their long historical development, tend to have an independent status in practice although they are clearly interlinked in theory. Each of these four scales is described separately and then the experimental evidence for their internal consistency is discussed.